Recently, attention is being paid to a Multiple-Input Multiple-Output (MIMO) system to maximize the performance and communication capacity of a wireless communication system. MIMO technology refers to a scheme capable of improving data transmission/reception efficiency using multiple transmit antennas and multiple receive antennas, instead of using a single transmit antenna and a single receive antenna. The MIMO system is also called a multi-antenna system. MIMO technology applies a technique of completing a whole message by gathering data fragments received via several antennas without depending on a single antenna path in order to form one whole message. Consequently, MIMO technology can improve data transmission rate in a specific range or increase a system range at specific data transmission rate.
MIMO technology includes transmit diversity, spatial multiplexing, and beamforming. Transmit diversity is a technique for increasing transmission reliability by transmitting the same data through multiple transmit antennas. Spatial multiplexing is a technique capable of transmitting data at high rate without increasing system bandwidth by simultaneously transmitting different data through multiple transmit antennas. Beamforming is used to increase a Signal to Interference plus Noise Ratio (SINR) of a signal by adding a weight to multiple antennas according to a channel state. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
Spatial multiplexing is divided into spatial multiplexing for a single user and spatial multiplexing for multiple users. Spatial multiplexing for a single user is called Single User MIMO (SU-MIMO) and spatial multiplexing for multiple users is called Spatial Division Multiple Access (SDMA) or Multi User MIMO (MU-MIMO).
The capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel may be divided into independent channels. Assuming that the number of transmit antennas is Nt and the number of receive antennas is Nr, the number of independent channels, Ni, is Ni=min{Nt, Nr}. Each of the independent channels may be said to be a spatial layer. A rank is the number of non-zero eigenvalues of a MIMO channel matrix and may be defined as the number of spatial streams that can be multiplexed.
In the MIMO system, each transmit antenna has an independent data channel. The transmit antenna may mean a virtual antenna or a physical antenna. A receiver estimates a channel for each transmit antenna to receive data transmitted from each transmit antenna. Channel estimation refers to a process of restoring a received signal by compensating for distortion of the signal caused by fading. Fading refers to a phenomenon in which signal strength abruptly varies due to multi-path time delay in a wireless communication system environment. For channel estimation, a reference signal that is known to both a transmitter and a receiver is needed. The reference signal may be referred simply to as an RS or may be referred to as a pilot according to applied standard.
A downlink reference signal is a pilot signal for coherent demodulation of a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), a Physical Downlink Control Channel (PDCCH), etc. The downlink reference signal includes a Common Reference Signal (CRS) shared by all User Equipments (UEs) in a cell and a Dedicated Reference Signal (DRS) for a specific UE. The CRS may be called a cell-specific reference signal and the DRS may be called UE-specific reference signal.
As compared to a legacy communication system supporting a transmit antenna, (e.g. a system according to LTE releases 8 or 9), a system having an extended antenna configuration, (e.g. a system supporting 8 transmit antennas according to LTE-A), needs to transmit a reference signal for obtaining Channel State Information (CSI), i.e. a CSI-RS, in a receiver.